dzhokhar tsarnaev. they see him plant a bomb. he's the one who the initial interest of the fbi was focused. then they noticed in going through additional videos, there's another man also involved. turns out to be his brother. and now we know what's become of the brother. we still don't know what's become of the younger man. but as they've now begun to learn more about what they were up to, it would appear they now believe there's a reason to think others were involved in planning the marathon bombing and they're still out there. >> a terrifying prospect and one that certainly has authorities in boston working very, very hard and asking the residents of that area to stay indoors, stay put while they try to get a handle on this situation. as we've been telling you this morning, it's almost too much to believe. a major american city essentially in lockdown. the public schools are closed. the transit system is suspended. there has been a no fly zone enacted over the area an lester holt is in watertown where a lot of this took place o